The present invention relates to a laser diode array and a method of manufacturing the same and more particularly to a phase-locked laser diode array which has a stable high output mode.
Due to the advance of optical communication technology and electronic industrial technology using semiconductor, opto-electronic era is coming near and the laser diode(LD) is spotlighted because of its good efficiency and reliability as well as its small size in data transferring or data recording.
Recently, the investigations on compound semiconductor of which properties are superior, for example GaAs which is not indirect transition type as Si or Ge but direct transition type, are in progress. This compound semiconductor has many superior properties such as high electron mobility, high frequency response, low power consumption, direct transition, and semi-insulativity to Si. Moreover, in the direct transition type semiconductor such as GaAs, the momentum is preserved during the recombination of electron and hole.
Therefore, the life time of charges injected toward the emitting region is so short (less than a few nanoseconds) that the emitting intensity is strong. And because the optical response for the charge of current input is very fast, the compound semiconductor is widely used as light source in the optical communication where fast modulation is required.
FIG. 1 is a vertical cross sectional view of a conventional VSIS(V-channeled Substrate Inner Stripe)-LD.
In the VSIS-LD, and P type GaAs current blocking layer 2 is formed on an N type GaAs substrate 1. Next, a V-channel is formed in a stripe shape by mesa-etching of the N-type GaAs substrate 1 and current blocking layer 2. On the current blocking layer 2, and N-type Alx Ga .sub.1 -x As layer 3, a P-type GaAs layer 4, a P-type Alx Ga .sub.1 -x As layer 5, and a P+-type GaAs layer 6 are sequentially stacked. The N-type Alx Ga .sub.1 -x As layer 3 is used as a first cladding layer, the P-type GaAs layer 4 as an active layer, the P-type Alx Ga .sub.1 -x As layer 5 as a second cladding layer, and the P+-type GaAs layer 6 as a cap layer.
The first cladding layer 3 fills the V-channel and is electrically connected to the N-type GaAs substrate 1. Also, a P-type electrode 7 is formed on the P+-type GaAs layer 6, while an N-type electrode 8 is formed beneath the N-type GaAs substrate 1. The normal planes to the V-channel are mirror facets.
The VSIS-LD generates light by the recombination of electrons and holes in the active layer 4 when voltage is supplied between the P-type electrode 7 and the N-type electrode 8. In order to limit the generated light inside the P-type GaAs layer 4, the refractive index of the P-type GaAs layer 4 must be larger than those of the N-type Alx Ga .sub.1 -x As layer 3 and the P-type Alx Ga .sub.1 -x As layer 5, whereas the energy bandgap of the P-type GaAs layer 4 must be smaller than those of the N-type Alx Ga .sub.1 -x As layer 3 and the P-type Alx Ga .sub.1 -x As layer 5.
Thus, the composition ratio X of the compound semiconductor must be in the range from 0 to 1, that is , 0&lt;X&lt;1. Also, the mirror facets normal to the V-channel reflect or emit the generated light without absorption.
Now, a method of manufacturing the VSIS-LD will be described.
After formation of P-type GaAs layer 2 on the N-type GaAs substrate 1, the V-channel is formed by mesa-etching. Also, the N-type Alx Ga .sub.1 -x As layer 3, the P-type GaAs layer 4, the P-type Alx Ga .sub.1 -x As layer 5, and P+-type GaAs layer 6 are sequentially formed on the N-type GaAs layer 2 by LPE (Liquid Phase Epitaxy) technique. Subsequently, the P-type electrode 7 of Au/Zn: Au is formed on the P+-type GaAs layer 6, while the N-type electrode 8 of Au/Ge: Ni: Au is formed beneath the N-type GaAs substrate 1. The P-type and N-type electrodes 7 and 8 forms the ohmic contacts with the P+-type GaAs layer 6 and the N-type GaAs substrate, respectively.
In manufacturing the VSIS-LD described above, there is a problem that two-step epitaxy is needed since the other layers 3,4,5, and 6 should be grown again after growing of the current blocking layer 2. In addition, in the mirror facets, traps due to crystal defects are generated easily. These traps make the energy bandgap so small that the mirror facets are broken because much light is applied onto the same at the time of high output. Moreover, the mode is unstable at the time of laser oscillating and the reproducibility of the mode is not good.